Graphite is widely used in nuclear reactors as moderator and structural material. Among present graphite preparation methods, air flow mill is considered to be qualified in the control of particle size and purity, and promising for future mass production. In this work, an opposed jet mill is designed to crush large graphite particles. The opposed jet mill accelerates the particles through two supersonic jet flows in opposite directions, and finally the particles collide in the crushing cavity. In order to estimate the performance of opposed jet mill, it is necessary to solve the coupling calculation of the compressible flow and the collision process of discrete particles. However, the research on calculating the compressible gas solid coupling problems is scarcely rare. In this paper, coupled CFD-DEM model is used to simulate the particle movement process with jet flows and accompanying jet in opposed jet mill. By comparing with experimental results, it is proved that these simulation results of the acceleration process of compressible gas through these nozzles and the collision process of the final two supersonic jet flows in the opposed-jet mill are accurate, with the accuracy model of the coupled CFD-DEM provided. The practice has proved that the contrastive flow mill has a broad application prospect in the production of graphite particles.

To better understand the flow features of pebble cluster in pebble bed, discharging of the pebble cluster were simulated by DEM. The pebble entangled cluster was composed of eight particles connected by rigid bonds and the simulated cluster models are divided into two types: axisymmetric u-particle and distorted z-particle. The simulation starts with the closed discharge outlet and the bonded clusters with different ID are randomly added from the entrance section. The pebbles fall freely and accumulate freely in the pebble bed. The discharge hole opens after all the pebbles being stationary for a period. Then the pebbles are discharged from the pebble bed under gravity. The discharging process is time-dependent bulk-movement behavior. There is not much mixing between layers on the boundary. The vertical end makes the packing loose, but also intensifies the interaction between particles due to entanglement. Consequently, the discharge features of pebble clusters of different included angles were quantified. The results show that the pebble discharging speeds depend on entanglement angle (α of u-particle and η of z-particle) and discharging outlet diameter. A large included angle may play the role of retarding or inhibiting the discharging flowrate. Therefore, the entanglement of particles component also always plays the key role of retarding the discharge.

The pebbles flow is a fundamental issue for both academic investigation and engineering application in reactor core design and safety analysis. In general, experimental methods including spiral X-ray tomography and refractive index matched scanning technique (RIMS) are applied to obtain the identification of particles’ positions within a three-dimensional pebble bed. However, none of the above methods can perform global bed particles’ position identification in a dynamically discharging pebble bed, and the corresponding experimental equipment is difficult to access due to the complication and high expense. In this research, the experimental study is conducted to observe the gravity driven discharging process in the quasi two-dimensional silos by making use of the high-speed camera and the uniform backlight. A mathematical morphology-based method is applied to the pre-processing of the captured results. After being increased the gray value gradient by the threshold segmentation, the edges of the particles are identified and smoothed by the Sobel algorithm and the morphological opening operation. The particle centroid coordinates are identified according to the Hough circle transformation of the edges. For the whole pebble bed, the self-programmed process has a particle recognition accuracy of more than 99% and a particle centroid position deviation of less than 3%, which can accurately obtain the physical positions of all particles in the entire dynamically discharge process. By analyzing the position evolution of individual particles in consecutive images, velocity field and motion events of particles are observed. The discharging profiles of 5 conditions with different exit are analyzed in this experiment. The results make a contribution to improving the understanding of the mechanism of pebbles flow in nuclear engineering.

Series of experiments are conducted in a single microchannel, where subcooled water flows upward inside a transparent and vertical microchannel. The cross section of the channel is rectangle with the hydraulic diameter of 2.8mm and the aspect ratio of 20. The working fluid is 3–15K subcooled and surface heat flux on the channel is between 0–3.64 kW/m 2 , among which two-phase instability at low vapor quantity may occur. By using a novel transparent heating technique and a high-speed camera, visualization results are obtained. The parameters are acquired with a National Instruments Data Acquisition card. In the experiments, long-period oscillation and short-period oscillation are observed as the primary types of instability in a microchannel. Instability characteristics represented from signals correspond well with the flow pattern. Moreover, effects of several parameters are investigated. The results indicate that the oscillating period generally increases with the heat flux density and decreases with inlet subcooling, while the effects of inlet resistance are more complex.

The influence of contact angle on bubble growth and detachment is investigated in this paper. The phase-change Lattice Boltzmann Method (LBM) which includes a SRT pseudo-potential LB model and a thermal LB model is used to simulate the flow boiling in vertical tube. To verify the correctness of the model, the coexistence curve obtained from the LBM simulations is compared to the analytical one. Then the relation between the diameter of bubble detachment and the wall superheat is compared with the empirical relation. The effect of contact angle on the bubble growth is investigated. The bubble growth with different contact angles is calculated. The bubble growth processes with different contact angles and the detached shape are shown in this paper. The bubble equivalent diameter and the length of contact line is investigated. The bubble equivalent diameter curve shows that bubble growth can be divided into two stages, initially isothermal growth stage, followed by isobaric growth stage. Bubbles show different characteristics at those two stages. The length of contact line curve shows there is a period of stagnation in the process of bubble growing up. This phenomenon can be explained by the theory of dynamic contact angle.

The effective thermal diffusivity and conductivity of pebble bed in the High Temperature Gas-cooled Reactor (HTGR) are two vital parameters to determine the operating temperature and power in varisized reactors with the restriction of inherent safety. A high-temperature heat transfer test facility and its inverse method for processing experimental data are presented in this work. The effective thermal diffusivity of pebble bed will be measured at temperature up to 1600 °C in the under-construction facility with the full-scale in radius. The inverse method presents a global optimal relationship between thermal diffusivity and temperature through those thermocouple values in the pebble bed facility. Furthermore, the robustness and uncertainty analyses are also set forth here to illustrate the validity of the algorithm and the corresponding experiment.

Helium purification and helium auxiliary system is one of important systems guaranteeing the safe operation of high-temperature gas-cooled reactor. In this system, wire mesh mist eliminator is one of the key components and used to separate waste water containing tritium, and remove moisture after reactor accident. Base on the ideal fluid model and packing pad model developing by Carpenter, A calculation model is presented for separation efficiency of mist eliminator. The calculation program “SEP-WMME” is developed based on the model. The calculation results fit well with experiment results. Theoretic analysis is carried out for the mist eliminator of HTR-PM helium purification system engineering validation test loop. The analysis shows the inlet velocity is an important parameter for mist eliminator. When the inlet velocity is above 3.0m/s, high separation efficiency will be obtained.

In high temperature gas-cooled reactors (HTRs), graphite is used as the main structure material. The side reflecter of the reactor core is composed by a pile of graphite bricks. In real operational condition of the reactor, both high temperature and fast neutron irradiation have great effect on the behavior of graphite components. The non-uniform distribution of temperature and neutron dose cause obvious stress accumulation, which greatly affects the security and reliability of the graphite components. In addition, high temperature and neutron irradiation make the properties of graphite change in evidence, and the changes are not linear. Such changes must be considered and simulated in the calculation, in order to predict the stress concentration condition and the reliability of the graphite brick correctly. A FORTRAN code based on user subroutines of MSC.MARC is developed in INET in order to perform three-dimensional finite element analysis of irradiated behavior of the graphite components for the HTRs. In this paper, the stress level and failure probability of graphite components are calculated and obtained under different in-core temperatures and neutron dose levels of the core side of brick. 400°C, 500°C, 600°C and 700°C are selected as the core side temperature, while the range of neutron dose is 0 to 1022n cm-2 (EDN). Different constitutive laws are used in stress analysis procedure. The impact of different temperature and neutron dose levels are discussed.

Flow drag and noise reduction technology is a multidisciplinary field currently relative to hydrodynamics, materials and physics. A comprehensive review of the literature on the principles, limitations and engineering applications of the technology was performed in this paper. Due to restriction of experimental conditions, it is rarely adopted to use to reduce flow resistance associated with noise for nuclear reactors. Assessment of effects of flow drag and noise reduction on circulation efficiency of cooling water and helium gas was done based on riblet structured surface, which would efficiently reduce the flow resistance and noise with high reduction rate and was available to realize for nuclear reactors. Results of this analysis showed that vortex induced in riblet grooves was the dominant factor resulting in flow drag and noise reduction on the riblet structured pipeline inner wall of cooling water and helium gas. Accordingly, higher circulation efficiency and heat transfer efficiency would eventually lead to better performance of nuclear reactors.

Micro arc oxidation (MAO) technology known as a newly surface treatment technology has got a widely application in the field of aviation, aerospace, automotive, electronics, and medical industry. Strength, toughness, hardness and corrosion of valve metal such as aluminum, magnesium, copper, zinc, zirconium and their alloys can be greatly improved by MAO technology. This paper tries to probe into the feasibility of using MAO technology in nuclear power industry. Aluminum and its alloys are used as structural materials such as the cladding of reactor fuel and all kinds of pipes in the low nuclear reactor. Zirconium alloys are widely used for the fuel cladding, cannula, catheter and other components of the fuel assemblies. Titanium and its alloys offer a unique combination of desirable mechanical properties which makes them to be the candidate materials for structural application in the field of nuclear energy. The surface of all these materials may be destroyed which increasing the risk of the nuclear accident due to the severe serving conditions. As a result, it is necessary to improve the corrosion and wear resistance behavior. With the urgent requirements of safety and durability of nuclear reactor, MAO technology must have a broad prospect in nuclear industry.

Natural circulation is an important process for certain advanced reactor’s main loop and passive heat removal systems. However, in marine conditions the thermo-hydraulic characteristics of natural circulation will change because of the ship motions such as inclination, rolling and heaving, which introduces extra body forces in to the system. In this paper, we conducted theoretical studies on the natural circulation behaviors in a symmetrical two-circuit loop under rolling conditions. A RELAP5/MOD3.3 code is developed based on the basic control equations and empirical formulas. Based on this code the natural circulation behavior under a larger range of rolling angle and period is also investigated. It is found that with the increase of rolling angle and decrease of rolling period, the magnitude of flow fluctuation increases. The fluctuation period of mass flow rate do not always consists with the rolling period under the comprehensive actions of complex body forces. Under the rolling with large angle and short period, the fluctuation period of flow rate is only half the rolling period.

Natural circulation systems are broadly used in marine environments. When accidents happen, these systems may work under inclined condition. In this paper, we conducted a series of experimental study on the thermohydraulics behavior of natural circulation in a symmetric two-circuit loop under the inclined angle from 0∼45°. A CFD model is also set up and predicts the results well in comparing with the experiments. Both experimental and numerical analysis show that with the increase of inclined angle, the total circulation flow rate decreases. When the loop inclines about the axis perpendicular to the circulation, one circulation is depressed while the other is enhanced; accordingly the disparity between the branch circulations arises and increases with the increase of inclined angle. The flow pattern of the circulation under larger inclined angle (45∼90°) condition is also studied by CFD model. At large inclined angle the circulation is mainly happens in one circuit. Also based on this model, the influences of flow resistance distribution and loop configuration on natural circulation are predicted. The numerical results show that to design the loop with the configuration of big altitude difference and small width are favorable to confine the influence of inclination; however too small loop width will cause sever reduction of circulation ability in large angle inclination.

Numerical simulations of air flow were carried out on non-smooth surface where microriblets were distributed uniformly at only one of the walls. An accurate numerical treatment based on k-ε turbulence model was adopted to study flow alteration and to analyze drag reduction and increasing mechanism on non-smooth surface. A modified calculation unit was used to estimate characteristics of flow at the reformed cells. With the microriblets aligned on the surface, the Reynolds shear stress was significantly decreased which was considered the dominant factor resulting in drag reduction. An additional force generating from the deviation of static pressure on the front and rear end of the riblet grooves caused pressure drag increasing exhibiting exponential growth with the flow rate, which was closely related to vortices induced by momentum transfer at the adjacent area of flow inside the grooves and the outer flow. Shear action at groove walls was greatly degraded due to the gradually variational velocity of vortices. Flow alteration on non-smooth surface compared with smooth surface was also analyzed in detail.

An active residual heat removal process is required by the gas-cooled reactor pebble-bed module plant being constructed in China, in order to improve the economical performance of the plant by shortening the restart duration. This process makes use of steam generator and start-up loop as heat sink, whose structure may suffer cold/heat shock while the sudden load of coolant or hot helium at the beginning. To achieve safety and reliability, transient analysis was carried out based on a one-dimensional mathematical model for steam generator and steam pipe of start-up loop. The calculation results show that steam generator should be discharged and pre-cooled; otherwise, boiling will arise and introduce a cold shock to the boiling tubes and tube sheet when coolant began to circulate prior to the helium. Additionally, in avoiding heat shock caused by the sudden load of helium, the helium circulation should be restricted to start with an extreme low flow rate; meanwhile, the coolant of steam generator (water) should have flow rate as large as possible. Finally, a four-step procedure with pre-cooling process of steam generator was recommended; sensitive study for the main parameters was conducted.

As well known, the very slow pebble flow (VSPF) exists in the pebble bed high temperature gas cooled reactor, which is regarded as the 4th generation of nuclear power plant, one of the most promising reactors in the 21th century. Unfortunately, we still know very little on the fundamentals of the VSPF, a specific type of granular flows. In general, a granular flow is a collection of a large number of discrete but closely packed solid particles. A granular flow may behave as either an elastic solid or a fluid, depending on the local stress conditions. A rapid granular flow behaves much more like a fluid rather than a solid collection, and sometimes can even be analogically analyzed by the kinetic theory. However, the very slow granular flow (VSGF) is showing its elastic solid behavior at any instantaneous time and fluid-like behavior in a long time. It can shear like fluid, dissipate energy by collision, and also it can support loads and form finite piling slopes like solid. It is more complicated than the rapid flow and cannot be easily analyzed solely by the fluid or kinetic theories. We regard the VSPF as a special VSGF. In the pebble bed under gravity, the pebbles flow out of the bed very slowly, almost one-by-one. Thus, the flow-out process is an intermittent or discrete process in time. We can regard each flow-out process as an “original disturbance” (OD), and the transfer process of OD throughout the bed as a “subsequent disturbance” (SD). The SD is decaying in time as the pebble collision is dissipative. Thus, the VSPF can be specifically defined as a special case in which the OD and SD processes are not overlapped. In other words, the effect of SD has already been dissipated almost completely before the next OD occurs. In this study, we just show some examples for the interesting phenomenon of VSPF both experimentally and numerically. Based on the OD and SD theory, we show the typical processes and basic characteristics of OD and SD to validate their existences and justify the definitions of VSPF by the OD and SD.